The present invention relates to an ink-jet printing apparatus and control method thereof and, more particularly, to an ink-jet printing apparatus and control method thereof capable of faithful reproduction of image data.
As recent years have seen advances in capabilities of an ink-jet printing apparatus, not only texts but also images are printed by the apparatus. In order to increase printing speed, an ink-jet printing apparatus generally comprises a group of nozzles where plural ink discharge orifices (nozzles) are integrally arranged for discharging one color and one density of ink. Furthermore, a plurality of such group of nozzles are respectively provided for different densities of one color of ink or for different colors of ink.
The ink-jet printing apparatus of this type performs printing by discharging ink from the group of nozzles while moving a printhead, having the group of nozzles, relative to a print medium. To move the printhead relative to a print medium, the following methods are adopted.
(1) A printhead, whose nozzles are arranged substantially in parallel to the direction X, is moved in the direction Y orthogonal to the direction X while a print medium is at rest, and printing is performed. Then, the print medium is intermittently moved by a predetermined distance in the direction X, and the printhead is moved in the direction Y for printing. Printing operation performed by repetition of the aforementioned processes is a so-called swath printing method.
(2) The nozzles are fixedly arranged in a manner such that they cover the entire length of a print medium in the direction Y, and printing is performed by moving the print medium at a constant speed in the direction X. This printing is a so-called full-multi printing method.
When an image is printed in these methods, a pixel is defined as a unit constructing an image. A pixel is not always constructed with a single dot (dot is formed by single ink discharge from a nozzle to a print medium), but may be constructed with a plurality of dots. In case of forming a pixel with a plurality of dots, these dots may be printed on substantially the same point, or may be printed on neighboring points. Either way, a pixel is defined by a predetermined rule. Image data to be printed is broken down to pixels by image processing means, and with respect to each of the pixels, a color to be printed and its density are determined by a predetermined rule. Printing is executed according to this determination. As mentioned above, one pixel may be constructed with a plurality of dots. In such case, different colors and densities of ink may be selected instead of one color and one density.
To faithfully reproduce tones of image data in image printing, a pseudo halftone processing method, e.g., dither processing, error diffusion processing and so on, is employed. In dither processing and error diffusion processing, a larger number of tones can be expressed if the number of tones expressed by one pixel is increased. A specific example of such printing method is described in Patent Application Laid-Open No. 10-324002.
More specifically, a group of nozzles capable of discharging different densities of ink are provided for one color, and printing is performed by superimposing ink from nozzles, selected from the aforementioned group of nozzles within a limitation set in advance for one pixel. By this, tones expressed by one pixel can be increased. For instance, a group of nozzles capable of discharging six types of densities are provided, and a pixel having 600 dpi is formed by superimposing ink four times or less. In this case, the pixel can express 50 tones or more. Moreover, if a pixel, constructed with neighboring 2xc3x972 points, is formed by superimposing ink the total of 16 times or less, the pixel can express 100 tones or more.
In the foregoing case, a rule, which associates the tone to be expressed with the method of ink superimposition, is determined in advance, and actual printing is performed according to the rule, i.e., which nozzles to use and when to discharge ink are determined. According to the determination, printing control means performs printing operation.
An example of superimposing ink is described hereinafter. First of all, a density is measured for a case of printing a pixel with each ink. Based on the measured values, densities obtained by superimposing ink are calculated, and a table associating each density with a combination of nozzles is prepared. A combination of nozzles, which achieves a closest density to the portion corresponding to a pixel of interest, is selected from the table. In the case of error diffusion processing, a difference is obtained between a density of the portion corresponding to the pixel of interest and a density in the table (density obtained by superimposing ink), and the obtained difference is diffused as an error.
When an image is printed by the aforementioned method, the amount of ink discharged from a group of nozzles is supposed to be constant. However, because of the printhead""s structural reason, the state of ink, or the state of driving mechanism for ink discharge and so forth, the amount of ink discharge is not constant in the strict sense. If printing is performed with the nozzles having such variations in the amount of ink discharge, an error is generated partially on a print image, and problems occur, e.g., a density unevenness is generated in the part which is supposed to have a uniform density, or a density variation supposed to show is submerged in noise and cannot be seen.
The similar problems occur not only when the amount of ink discharge is not constant, but also when ink density slightly differs depending on the position in the group of nozzles. In addition, the similar problems occur in a case where a printing pitch, which is supposed to be uniform, becomes partially larger or smaller than a predetermined density due to uneven directions of ink discharge.
This is problematic in a case where faithful reproduction of an original image density is required, for instance, a medical image. A monochrome image is usually printed on a medium for medical evaluation. The reason thereof is that human eyes have a higher density resolving power for a monochrome image. Therefore, the amount of data that can be recognized by human eyes is large in a monochrome image. Furthermore, it is known that a density resolving power, that can be recognized by human eyes, is higher when using a transmission-type print medium rather than a reflection-type print medium. It is said in general that the density resolving power of human eyes is about 8 bits for a color image and 10 to 11 bits for a monochrome transmission image. Medical X-ray photographs or CT/MRI images printed on transmission-type media are read actually to the limit of the human density resolving power for providing information for diagnoses. In the field that requires such high quality in images, the aforementioned slight density difference causes unevenness or granularity in images.
In order to solve these problems, a so-called shading correction has been proposed. In the shading correction, test pattern printing is performed in advance at a constant density (density which should be constant) using a number of ink combination patterns, then densities of the test patterns are read by a scanner, and density unevenness is obtained to correct image data subjected to printing. (Note: the xe2x80x9ctest pattern printing . . . at a constant density (density which should be constant)xe2x80x9d means that the density should be constant if the density in the printing portion has a value as designed. In reality, the density becomes slightly uneven because of an error caused by various factors.) However, if the shading correction is applied to the aforementioned printing method, in which a pixel is constructed by multiple superimposition of dots, the number of combination patterns becomes substantially large, making the processing complicated (for instance, the case of selecting up to four types of densities out of six types of different densities with redundancy). Furthermore, if image data of an original image is corrected, then correction is necessary for the corrected density again, and the correction may not converge.
The present invention has been proposed in view of the above problems, and has as its object to enable correction of density unevenness in each nozzle and to enable high quality image formation.
Furthermore, an object of the present invention is to enable correction of density unevenness in each nozzle for printing an image by constructing a pixel with plural types of ink superimposition, thereby enabling high quality image formation.
According to the present invention, the foregoing object is attained by providing an ink-jet printing apparatus comprising: a printhead including a group of nozzles constituted by a plurality of nozzles; storage means for storing a density table indicating a print density of ink discharged from each nozzle constituting the group of nozzles; determination means for determining a nozzle to be driven for discharge in the group of nozzles; and density compensation means for obtaining from the density table a print density corresponding to the nozzle, determined to be driven for discharge by the determination means, and performing compensation control of a print density based on the obtained print density.
According to the present invention, the foregoing object is attained by providing an ink-jet printing apparatus for printing a pixel with dots formed with plural types of ink, comprising: a printhead including a plurality of a group of nozzles provided for a plurality of types of ink, each of the group of nozzles constituted by a plurality of nozzles corresponding to one type of ink, storage means for storing a density table, indicating a print density of ink discharged from each nozzle constituting the group of nozzles, with respect to at least one group of nozzles among the plurality of the group of nozzles of the printhead; determination means for determining a nozzle to be driven for discharge in the plurality of the group of nozzles, with respect to a pixel of interest based on image data; calculation means for obtaining a print density corresponding to the nozzle, determined to be driven for discharge by the determination means, with reference to the density table and calculating a print density of the pixel of interest; and density compensation means for performing compensation control of a print density based on the print density of the pixel of interest calculated by the calculation means and a print density of the pixel of interest designated by the image data.
According to the present invention, the foregoing object is attained by providing a data processing apparatus generating print-driving data for a printing apparatus which performs printing by using a printhead including a group of nozzles constituted by a plurality of nozzles, comprising: storage means for storing a density table indicating a print density of ink discharged from each nozzle constituting the group of nozzles; determination means for determining a nozzle to be driven for discharge in the group of nozzles; density compensation means for obtaining from the density table a print density corresponding to the nozzle, determined to be driven for discharge by the determination means, and performing compensation control of a print density on the determination means based on the obtained print density; and output means for outputting data, indicative of the nozzle to be driven for discharge, determined by the determination means, as the print-driving data.
According to the present invention, the foregoing object is attained by providing a data processing apparatus generating print-driving data for a printing apparatus which prints a pixel with dots formed with plural types of ink by utilizing a printhead including a plurality of a group of nozzles provided for a plurality of types of ink, each of the group of nozzles constituted by a plurality of nozzles corresponding to one type of ink, comprising: storage means for storing a density table, indicating a print density of ink discharged from each nozzle constituting the group of nozzles, with respect to at least one group of nozzles among the plurality of the group of nozzles of the printhead; determination means for determining a nozzle to be driven for discharge in the plurality of the group of nozzles, with respect to a pixel of interest based on image data; calculation means for obtaining a print density corresponding to the nozzle, determined to be driven for discharge by the determination means, with reference to the density table and calculating a print density of the pixel of interest; density compensation means for performing compensation control of a print density on the determination means based on the print density of the pixel of interest calculated by the calculation means and a print density of the pixel of interest designated by the image data; and output means for outputting data, indicative of the nozzle to be driven for discharge, determined by the determination means, as the print-driving data.
Still further, the present invention provides a control method of the above-described ink-jet printing apparatus and a data processing method using the aforementioned data processing apparatus.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.